Basis for deciding Stress critical lines with PDF

Stress analysis is a complex task and in any process unit, there are a huge number of lines exist which run from one location to another. Analyzing all lines will take a lot of time which in turn will increase the engineering time and corresponding cost. So every engineering organization in this field has set up some guidelines for deciding which lines are to be stress analyzed using software (Caesar II, Autopipe, Caepipe, START-PROF, or Rohr II).

What is a Critical Line List or Stress Critical Line List?

Stress Critical Line List or SCLL is a listing of all critical lines requiring attention from a piping stress engineer during the piping design phase. It is the the piping stress engineer’s responsibility to prepare one Critical Line List (CLL) by isolating the non stress critical lines from the stress critical line based the criteria provided in the stress analysis specification. All those critical lines are then categorized into some number of stress systems so that stress system number can be easily found out from the generated critical line list. The number of stress critical lines in each stress system is decided based on the engineering judgement of the stress engineer. Stress Critical Line List is an important deliverable from piping stress team. The main inputs required from the stress critical line list preparation are:

  • Process Line List
  • Process P&ID or PEFS, and
  • Flexibility Specification or Stress Analysis Design Basis.

Ideally, the critical line list should be updated as and when the process line list is updated. But to reduce frequent changes many organization prepares only three revisions of stress critical line list. Those:

  • Preliminary Stress Critical Line List during the start of the project upon receipt of the first set of line list and P&ID
  • Advanced Critical Line List just before 60% model review and
  • Final Stress Critical Line List after 90% model review.

Types of Stress Critical Lines

Stress Critical Lines are normally divided into few groups for deciding critical lines. Those are:

  • Equipment Critical Lines: Lines connected to Rotary Equipment and Critical Static equipment falls in this category. For example, lines connected to pumps, compressors, and turbines are by default stress critical and require analysis using the software.
  • Support Critical Lines: Lines for which engineered supports are required falls in this category. Example: Supports requiring spring hangers, Pipes with SS or SDSS material where normal CS supports can not be used, etc.
  • Relief Critical Lines: Pipes experiencing relief loads comes into this category. For example, the line connected to pressure safety valves, rupture disk, etc.
  • Material Critical Lines: Pipes made from SDSS, Non-Metals like GRE, FRP, Aluminum alloy, etc falls in this category.
  • Service Critical Lines: Piping systems carrying category M fluid service, hazardous fluid service, severe cyclic condition, etc. falls in this category.
  • Temperature Critical Lines: Lines carrying fluids having high temperature comes into this group.

The basis for deciding Stress critical lines

The main factors which decide stress critical lines for preparing a critical line list are as follows:

  • Line design/operating/upset temperature
  • Equipment connection
  • Pipe and Equipment material
  • Pipe condition
  • Pipe thickness
  • Design/Upset pressure
  • Fluid Category
  • Severe cyclic condition

Every organization has its own guidelines and the guidelines vary from project to project. The following write-up will provide a few criteria for deciding stress critical lines. This is only an idea of how differentiation occurs. The user is requested to check project-specific documents for use in any project. Mostly the critical lines for which stress analysis is to be performed by formal computer analysis consists of the following lines:

  • All Pump (Centrifugal-API/ANSI, gear pump, Screw pump) suction and discharge piping (4 inches and larger).
  • Centrifugal Compressor inlet and outlet piping.
  • Lines to and from steam generators.
  • Reciprocating pump and compressor suction and discharge piping.
  • Piping requiring expansion joints or other proprietary expansion devices.
  • Steam and Gas Turbine inlet and outlet piping.
  • Air Cooler inlet and outlet piping (3 inches and larger).
  • Process Heater inlet and outlet piping
  • Lines classified as category M as per ASME B31.3.
  • Piping subjected to high cyclic temperature conditions.
  • All jacketed lines.
stress critical
  • Lines that require nozzle load compliance as stipulated per applicable codes or equipment Vendor allowable (Heat exchanger, Pressure Vessel Connected systems).
  • Lines subject to dynamic loading (relief lines, line with large pressure drop at control valves, surge pressure, slug flow, churn, two-phase flow, water hammer, flashing, etc.)
  • All Fiberglass, aluminum alloy, refractory, or elastomer lined piping.
  • All piping systems connected to FRP, plastic, glass-lined steel, or brittle equipment
  • Lines subjected to non-thermal movements (Expected differential settlement between structures, structure-equipment, etc., process equipment growth, header growth, tower growth or other significant displacements, etc.)
  • All lines 8” and larger operating above 150 deg. C (300 deg. F) and greater.
  • All lines 20” and larger operating above 80 deg. C (200 deg. F) and greater.
  • All lines 36” and larger.
  • All lines operating below -45 deg. C (-50 deg. F) which requires special “cold” supports.
  • All plastic lined piping systems. Special attention shall be given to adding enough additional supports to limit the external forces and moments in the flange connections to avoid an extra risk of flange leaks
  • Lines with special design requirements
  • All Safety pressure-relieving systems 4 inches and larger (not including thermal reliefs)
  • Lines judged by the lead piping engineer/stress engineer as not having sufficient inherent flexibility
  • In addition, the piping effects of other conditions such as temperature gradients that could cause thermal bowing or where piping is connected to equipment with significant thermal growth may warrant detailed computer analysis.
  • For thin wall piping, if the D/T ratio exceeds 100, the following requirements are applicable:
    • The design and support of piping systems using this specification should be reviewed by a stress engineer. Support and spans of thin wall piping systems are not covered by current Project practices and therefore must be designed for each application.
    • Stub-in connections per 304.3.2 thru 304.3.4 of ASME B31.3, are not allowed for run pipe with D/T greater than or equal to 100 and the branch diameter is greater than one half of the header diameter.
  • Lines connected to non-ferrous equipment.
  • Underground process lines with more than a 30-degree difference between design and ambient temperature.
  • All vertical lines connected to vertical vessels that require pipe supports or guides from that vessel.
  • All lines 4 inches and larger subject to external pressure or vacuum conditions.
  • All lines subject to vibration, as specified by Process, due to high-velocity flow, high-pressure drop, water hammer, or mixed phase flow.
  • All lines that are connected to equipment constructed of thermoset or thermoplastic materials or that are glass, refractory, or elastomer lined.
  • All pressure containing non-metallic lines.
  • All flare line headers
  • Lines for which an Alternative Leak Test has been specified.

Many organizations have the practice of dividing these critical lines into three groups based on their criticality:

  • Highly critical lines or group C1 lines: Must be reviewed thoroughly using stress analysis software.
  • Moderately Critical lines or group C2 lines and
  • Lower critical lines or group C3 lines

Few more useful Resources for You…

Resources related to Piping Materials
Articles related to Piping Layout and Design
Basics of Piping Stress Analysis

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Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

8 thoughts on “Basis for deciding Stress critical lines with PDF

  1. Thanks for sharing the info .
    very helpful for begineers like me

    to clear my concept one question

    8″ pipe less than 150 c temp , is it required to do analysis.???

  2. Thank you for explaining how stress lines are determined via pipe condition, thickness, and equipment material. I imagine it’s important to thoroughly understand the weakness of any pipeline and reinforce them in whatever way possible. Suspending piping probably makes it even more important to make sure the pipes are properly supported using hangers or other methods.

  3. Hi,

    I was through your blog and answers ,these are in depth analysis.
    I want to know why some linesare not analysed for flexibility as per formulae of
    D.y/(L-U)2 <=K.

  4. Can you help me in this issue, As we are assigned on a chilled water piping stress analysis. we have selected pipes 6 ” and above for formal Stress analysis, based on our practice( critical line list), and code selected was 31.9, Now Consultant is asking a question why do we not included smaller pipes for analysis, they are expecting reply with reference from ASME codes , Does codes tell directly anything about the selection of critical line list ? What is then the reference for selecting critical lines based on pipe size? Alex Matveev

  5. Sir I am 33 years old ., I have 7 years experience in chemical cleaning industry but I want change my field in piping design engineer , please give me some idea for I get job this field.

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